This proposal is device design driven. It is based on a recent paper by the PI and his colleagues: J. S. Hyde, H. S. Mchaourab, T. Camenisch, J. J. Ratke, R. W. Cox, and W. Froncisz, EPR detection by time-locked sub- sampling, Review of Scientific Instruments 69, 2622-2628, 1998. On the one hand, this paper established feasibility for the concepts, while on the other, a number of engineering issues arose that require additional research and development. The broad long-term goal is to develop a new general purpose electron paramagnetic resonance (EPR) spectrometer based on modem signal acquisition and signal processing methods and also on very high speed personal computers with very large memory capacity. It is expected that all EPR spectrometers will eventually be based on the ideas introduced in this proposal. Features of time-locked sub-sampling (TLSS) detection include (i) increased sensitivity relative to conventional spectrometers by about a factor of 3; (ii) simultaneous collection of all harmonics of the response of the spin system to field modulation, both in- and out-of-phase for both dispersion and absorption, limited only by adjustable parameters; (iii) broadbandedness, making it ideal for detection of spectra from transient species; (iv) totally digital detection and storage of data without initial signal averaging. The three specific aims involve (i) construction of the instrument; (ii) engineering studies of the instrument with additional upgrades throughout the funding period; and (iii) three biomedical applications made possible by TLSS detection. These applications are not only of interest in themselves, but also will be powerful tests of the capabilities of the instrument. They address three broad areas of biomedical EPR research: (i) nitrogen ligation in copper-containing enzymes; (ii) kinetics of peroxynitrite chemistry; and (iii) measurement of fluctuations in protein structures. EPR spectroscopy is a general approach to the study of the structure and dynamics of proteins and macromolecular assemblies. It is the only method that can detect the formation of free radicals in biology unambiguously. It contributes to health-related research at the basic fundamental level.